Raw digital images generated by imaging devices, e.g., digital cameras, that use sensor arrays, such as a complementary metal oxide semiconductor (CMOS) sensor arrays, may be degraded by one or more types of distortion. The distortions can include, for example, spectral crosstalk, electrical crosstalk, optical crosstalk, and sensor shading.
Spectral crosstalk may be caused by imperfect color filters that pass some amount of unwanted light of other colors. Spectral crosstalk may cause colors in a generated image to be unsaturated as compared to the color of the originally photographed object or scene. One approach for correcting spectral crosstalk is to apply a color correction matrix to the raw image data that compensates for the spectral crosstalk.
Electrical crosstalk may result from photo-generated carriers within the sensor array of the imaging device moving to neighboring charge accumulation sites, i.e., neighboring sensors within the sensor array. Electrical crosstalk is a function of the underlying sensor images that form the sensor array of the imaging device. As a result, the distortion caused by electrical crosstalk is uniform across the entire image. One approach for correcting electrical crosstalk is to apply a predetermined correction factor to each sensor response value included in the raw image data.
Optical crosstalk is the result of more complex origins. As described in greater detail below, color filters included in the optical sensor unit of an imaging device are placed some distance from the pixel surface due to metal and insulation layers. As a result, light coming at angles other than orthogonal may pass through a color filter element for a pixel, yet may pass diagonally to an adjacent pixel sensor rather than to the pixel sensor associated with the color filter element through which the light passed. Depending on the focal ratio of the lens, the portion of the light absorbed by neighboring pixel can vary significantly. For this reason, optical sensor units may include an array of micro-lenses layered over the array of color filter elements in order to redirect the light in the direction of the intended pixel sensor. However, longer wavelengths, e.g., red light, are more difficult to bend than shorter wavelengths of light, e.g., blue light, so optical crosstalk may occur despite the use of such micro-lenses.
The level of the optical crosstalk depends on the wavelength of light and the angle of incidence on the light on the individual pixels of the sensor array. Since the angle of incidence is related to pixel position, optical crosstalk is non-uniform for the whole image. Blue light is more easily bent by a micro lens and, therefore, is efficiently directed onto the sensor at the bottom of the pixel, while red light is not easily bent by a micro lens and, therefore, may leak to adjacent pixels. As light travels from a center focused lens onto the sensors in a sensor array, the angle of incidence of light on the respective pixels increases with the distance of the pixel from the center of the image, thereby increasing likelihood that light with longer wavelengths may fall onto an adjacent pixel's sensor.
As a result of optical crosstalk, the center of an image may appear brighter and redder and the surrounding portions of the image may appear darker and bluer. Although optical crosstalk may be partially corrected for using existing optical crosstalk correction, such techniques are often inadequate.
Sensor shading is the result of pixels closer to the center of an image shading pixels further from the center of the image. As described above with respect to optical crosstalk, color filters included in the optical sensor unit of an imaging device are placed some distance from the pixel sensors due to metal and insulation layers. As a result, light coming at angles other than orthogonal may be shaded from reaching its intended pixel by the metal and insulation layers above pixels closer to the center of the image. Further, lens shading is caused by the physical dimensions of a multiple element lens. Rear elements are shaded by elements in front of them, which reduces the effective lens opening for off-axis incident light. The result is a gradual decrease of the light intensity towards the image periphery. Although sensor shading and lens shading are not wavelength dependent, as is optical crosstalk, such sensor shading results in a similar form of radially increasing distortion, i.e., radial fall-off, by decreasing the amount of light reaching the pixels in the sensor array at locations further from the center of the image.
As a result of the combined effects of optical crosstalk and shading, pixels at the center of an image will experience minimal or no leakage and shading, but pixels further from the center of an image will experience increasing levels of leakage, especially for red pixels, and increasing levels of shading.